The present invention relates generally to boilers, such as pulp mill recovery boilers and, more particularly, to studs welded to water-carrying heat-exchange tubes of such boilers for heat exchange and protective purposes.
In a pulp mill where paper is made, wood chips processed from debarked logs are cooked in a soda solution in a high pressure vessel known as a digester. The soda solution at high temperature and pressure dissolves resins (lignin) binding cellulose fibers in the wood chips. The cellulose fibers are separated, washed, bleached and further processed to manufacture paper, or for other applications.
After cellulose fiber separation, what is left is an aqueous solution The aqueous solution is concentrated by evaporation to a concentration of approximately one-third water. The rest is combustible resin (lignin) and chemicals which can be recovered. Up to 98% of the chemicals used in the process can be recovered. Moreover, the resins constitute an excellent fuel.
Universal practice is to thus concentrate the solution by evaporation and make various chemical adjustments to form what is known as black liquor, and then to burn the black liquor as fuel in a recovery boiler. A recovery boiler is a large structure, perhaps fifteen stories high, and thirty to forty feet (9.1 to 12.2 meters) wide. The lower portion of a recovery boiler where combustion occurs is known as the furnace, and is the hottest part The walls of a recovery boiler are waterwalls formed of water-carrying tubes which are heated by the combustion process to usefully generate steam.
Within the recovery boiler, the resins constituting part of the black liquor are burned to produce heat and waste gases, while the chemicals such as soda form a molten residue known as smelt, which is recovered.
Advantages of this process are that burning the resin part of the black liquor as fuel generates steam which typically provides more than half of a mill's energy requirements, and that nearly all of the chemicals used in the process of cooking wood chips are recovered in the form of the smelt. Also, processing the black liquor in a recovery boiler in this manner solves what otherwise would be a serious environmental concern in disposing of the aqueous solution which remains after cellulose fiber separation.
A complicating factor in this process is that the smelt temperature is approximately 2100.degree. F. (1149.degree. C.). The smelt and gases within the recovery boiler are chemically highly active at these temperatures. Also, during boiler operation, the black liquor is typically sprayed from a number of nozzles directly against the walls of the lower, furnace portion of the boiler. Thus, the carbon steel water-carrying tubes are subject to corrosion and eventual destruction, which would necessitate extremely expensive rebuilding of the boiler. Moreover, failure of the water-carrying tubes is potentially catastrophic as an explosion can occur if water within the tubes comes into contact with the 2100.degree. F. (1149.degree. C.) smelt. This corrosion process is described in detail in a technical paper by J. A. Dickinson, M.E. Murphy and W. C. Wolfe (Babcock & Wilcox) entitled "Kraft Recovery Boiler Furnace Corrosion Protection", presented to TAPPI Engineering Conference, Atlanta, Ga., Sept. 28 through Oct. 1, 1981.
A common practice in recovery boilers, particularly in the furnace portion, is to employ for corrosion protection a multiplicity of cylindrical studs, analogous to heat-exchange fins. Each stud has a base or attachment end welded to the external surface of a water-carrying tube, and an exposed or tip end projecting radially outward from the tube. Conventional studs are made of low carbon steel and, when new, are typically 3/8 inch (0.95 cm) or 1/2 inch (1.27 cm) in diameter, and 3/4 inch (1.91 cm) in length. Studs typically are applied at a density of ninety studs per lineal foot (30.5 cm) of three-inch (7.62 cm) diameter water-carrying tube. A recovery boiler may have anywhere from 100,000 to 1,000,000 studs in total. The Dickinson et al paper referenced above analyzes the effects of various stud diameters, arrangements and densities.
The studs serve a number of important functions. One function is to aid heat exchange in the manner of heat-exchange fins between the 2100.degree. F. (1149.degree. C.) smelt and the outer walls of the water-carrying tubes, which typically have a temperature in the range of approximately 550.degree. F. (288.degree. C.) to 600.degree. F. (316.degree. C.). Closely related to the heat-exchange purpose, another purpose of the studs is to promote rapid cooling of the molten smelt, which solidifies to form what is known as a frozen smelt layer. The frozen smelt layer advantageously serves as a refractory layer preventing direct contact between the hot smelt and the metal walls of the water-carrying tubes. The frozen smelt layer minimizes the amount of corrosive gases penetrating to the tube surface, and provides a thermal insulating effect. The studs thus can become partially or entirely embedded in the frozen smelt layer, with heat transfer occurring in large part through the tips of the studs. The studs may be viewed as an interface which prevents direct contact between the hot smelt and the surface of the tubes.
During boiler operation, the black liquor is sprayed towards the walls and burned, transforming the organic fractions of the black liquor into exhaust gases, while the inorganic fractions melt and flow downward, accumulating in the furnace bottom, and flowing away through spouts into a dissolving tank. Combustion normally starts as black liquor is atomized and emerges as droplets from a nozzle, which droplets travel within the furnace towards the walls. Once the droplets reach the walls, combustion is essentially complete, and the smelt accumulates among the studs. If the cooling conditions are sufficient, some of the smelt stabilizes forming the frozen smelt layer. The process is continuous and dynamic; thus, the frozen smelt layer frequently falls away as a paste, momentarily exposing the surface of the carbon steel water-carrying tubes. The tubes are then immediately recoated with a new smelt layer, which becomes a frozen smelt layer when properly cooled down.
Another significant function of the studs related to this process is an anchoring effect, which promotes the building up and maintenance of the frozen smelt layer to protect the water-carrying tubes.
Even while completely or partially embedded in the frozen smelt layer, the studs continue to transfer heat. Inside the frozen smelt layer, the temperature is much lower than the 2100.degree. F. (1149.degree. C.) smelt temperature, reducing the corrosion rate of the carbon steel boiler tubes.
A characteristic of these studs, which has both advantages and disadvantages, is that they are sacrificial bodies and are consumed during operation. One disadvantage is that the studs are less able to satisfy their above-discussed purposes as they are gradually consumed, and eventually must be replaced, with attendant cost in terms of both direct replacement expense and mill downtime. Consumption of the studs presents a significant threat to boiler safety, since the capability of providing a frozen smelt layer no longer exists if the studs are completely consumed. Studs are replaced when they have worn from their original 3/4 inch (1.91 cm) length to 1/4 or 3/16 inch (0.64 or 0.48 cm). Typically they are consumed within one to two years. Replacement studs are directly attached to nubbins of worn studs by electric arc welding. A variety of specific welding techniques are employed, involving for example gas protection or a ceramic ring. Studs can be replaced at a rate of 50,000 to 100,000 per day. Sometimes, as many as 250,000 studs are replaced at a time.
Another disadvantage relates to the fact that conventional studs do not maintain a cylindrical shape as they wear. Rather, they assume a conical shape as the circular edge at the end of each cylindrical stud receives heat input at a relatively higher rate than it can be conducted through the stud. Thermal analysis can demonstrate that the circular edge is at a higher temperature than the rest of the stud, which causes a relatively rapid collapse of the edge as the stud assumes the conical or rounded shape. Temperature varies in the axial direction along each stud, with the stud base (attached to the tube) having the lowest temperature, close to the temperature of the tube wall itself. The new profile eventually stabilizes as heat flow reaches a balance between incoming heat and transferred heat. This rounded or conical profile is not as effective as a conical, flat-tipped stud in anchoring the frozen smelt layer. Thus, the studs become less effective in anchoring the frozen smelt layer than they otherwise would be.
A particularly significant advantage of the sacrificial nature of the studs is that the wear patterns of the studs provide valuable information regarding combustion conditions within the recovery boiler. It is a major engineering challenge to spray the fuel (black liquor) evenly within the furnace for even heating. Moreover, unevenness in heating is caused by the manner in which combustion air is introduced into the furnace. Other variable factors include water flow conditions in general, and internal depositions within the tubes. As a result, "hot spots" are common in boilers, which require careful monitoring inasmuch as the premature failure of even one tube could have disastrous consequences. Thus, the wear patterns of the studs at different points within the boiler is normally closely observed at periodic intervals both to maintain proper operating conditions, and to determine the need for preventative maintenance.
Another advantage of the sacrificial nature of the studs is that the sacrificial wearing of the studs tends to limit the wear on the water-carrying tubes, since these water-carrying tubes are not directly exposed to the 2100.degree. F. (1149.degree. C.) smelt temperature.
A different prior art approach to protecting water-carrying tubes in a recovery boiler is to employ unstudded composite tubes. Composite water-carrying tubes are made of two different materials, extruded together. The external part is made of stainless steel, which protects the inner portion, made of low carbon steel. Direct contact between the inner carbon steel tube and the smelt is accordingly prevented. In principle, if the tube skin is able to withstand the chemical attack of the smelt, then studs are not necessary. There is a potential for relatively long life. (Since stainless steel has four times the thermal resistance of carbon steel, it is unlikely that an all stainless steel tube thick enough for structural soundness would have sufficient heat transfer capability. Accordingly, a relatively thin stainless steel skin is provided over the carbon steel tube.)
Composite tubes, however, have their own disadvantages. A significant disadvantage is that the composite tubes do not wear in a manner which clearly indicates "hot spots" in a boiler which, as discussed above, result from variable factors such as fuel distribution, air distribution, water flow conditions, internal deposition inside the tubes, as well as other factors. Closely related to this, there is no warning whatsoever should the protective stainless steel skin become locally worn away, exposing the carbon steel inner tube portion. Catastrophic failure can accordingly occur with no warning.
Also, the replacement of carbon steel tubes with composite tubes is an extremely expensive process, with an attendant lengthy downtime. Similarly, repairing a composite tube is difficult.
While the foregoing discussion has been in the context of studs for tubes in pulp mill recovery boilers, it will be appreciated that related (but not necessarily identical) considerations apply in the case of other boiler applications. While a pulp mill recovery boiler is a particularly corrosive environment, other adverse situations include abrasive environments. For example, waste disposal incinerators can be of similar construction, and in such incinerators abrasive particles and various solid materials may be directed towards the water tubes. Other examples are oil- or coal-fueled cyclone boilers for power generation, and various types of furnaces used in industrial processes.